I. Field of the Invention
The present invention relates to a device for detecting the presence of an intruder listening device on a telephone line or the like.
II. Description of the Prior Art
In order to maintain compatability between telephone systems throughout the world, the telephone lines for virtually all telephone systems share common electrical characteristics. These electrical characteristics include the impedance of the line, the electrical resistance of the line, the line voltage and the closed loop current of the line. Furthermore, these electrical characteristics will vary depending on whether the telephone receiver is either on the hook or off the hook.
Most types of intruder listening devices, i.e., wire taps, alter one or more electrical characteristics of the telephone line when the intruder listening device is attached across the telephone line. Thus, measurement of the electrical charactertistics of the telephone line will reveal the presence or absence of most types of intruder listening devices.
There have been previously known devices which, when attached to the telephone lines, measure certain electrical characteristics of the telephone line in order to detect an intruder listening device. These previously known testing devices, however, are both bulky and expensive in construction. Furthermore, such devices are only periodically used to test the telephone lines for intruder devices so that an intruder listening device on the telephone line will remain undetected until the next scheduled test of the telephone line.